Photothermal deflection spectroscopy is a new measurement technique and has been disclosed, for example, in Analytical Chemistry, 63, 217 (1991).
The principle of photothermal deflection spectroscopy is as follows:
A sample is irradiated with an excitation beam of specified wavelength. As a result, compounds having photothermal conversion activity corresponding to that wavelength generate heat and the quantity of the heat produced is measured by the deflection of a probe beam which passes through the sample. The amount of the target compound present in the sample is determined from the amount of heat produced.
The application of this technique to immunoassays using a compound having photothermal conversion activity as a marker is the same kind of marker as the radioisotope used in radioimmunoassays, is known from Analytical Science, 7, 1387 (1991). However, immunoassays using a carrier capable of amplifying the photothermal deflection effect are not known.
In general, immunoassays are classified as either heterogenous or homogeneous. In a heterogenous assay, after the substance to be assayed is reacted with an antigen or an antibody having a marker bound thereto, the bound and the free markers are separated (B/F separation), and washed prior to measurement. Homogeneous assays are carried out without any B/F separation, but have so far not yet been used in immunoassays using photothermal deflection spectroscopy as the measurement technique.
Homogeneous immunoassays have the advantage that the measurement can readily be carried out using simple procedures after only a short period of time. However, in the homogeneous assay the sample will contain not only the reacted marker (i.e. bound marker) but also unreacted marker (i.e. unbound marker). The detected signals from the sample will therefore include a signal (S), derived from the bound marker, and a signal (N), derived from the unbound marker. The signal-to-noise (S/N) ratio in a homogeneous assay is therefore low. For this reason, homogeneous immunoassays are of low precision, and there is a great demand for improvement.